The invention relates to a process for operating the brake arrangement of a vehicle, which brake arrangement comprises an electrically controllable service brake system, which is set to generate brake forces independently of driver actuation, and which comprises an electrically controllable parking brake system, which is set to generate brake forces and maintain these forces.
The service brake system enables the driver to gradually reduce the speed of the vehicle during its operation or to bring the vehicle to a standstill. Modern vehicles are today equipped with an electrically controllable service brake system in order to enable brake functions which are independent of driver actuation, i.e. automatic brake functions, such as drive slip control (ASR) or driving dynamics control (ESP) to be carried out in addition to the anti-locking control function (ABS). To this end, the service brake system comprises, in known manner, a correspondingly constructed electrohydraulic control unit, an electronically controllable brake booster or it is constructed as a so-called “brake-by-wire” system.
By contrast, the parking brake system also enables the vehicle to be held on a road with an incline, and particularly kept stationary when the driver is absent, using mechanical means. The aim today is also to construct the parking brake system such that its electrically controllable, as already known for example as an “electric parking brake (EPB)”. With this, the driver only needs to actuate an electrical control element by way of which at least one electromechanical actuating unit is activated to actuate the actual wheel brakes. Whilst, with the so-called “cable puller”, only a central electromechanical actuating unit is used, which replaces the conventional parking brake actuating element (lever or pedal) and acts on the wheel brakes concerned in conventional manner by way of cables, there are also EPB systems with generally two peripheral electromechanical actuating units which are constructed or integrated directly on the wheel brakes concerned.
Therefore, DE 101 50 803 A1 discloses a wheel brake which can be hydraulically actuated in conventional manner for the functional range of the service brake system. To this end, the wheel brake has a brake piston which acts on at least one friction lining and can be displaced by means of a hydraulic pressure introduced into a hydraulic chamber to clamp the at least one friction lining against a rotary element connected in torsion-resistant manner to a wheel of the vehicle in order to generate a brake force. So that the functions of the parking brake system can also be carried out, an electromechanical actuating unit is furthermore integrated in the wheel brake, and this has a gear unit driven by an electric motor, which acts on the brake piston in order to clamp and mechanically fix this against the rotary element connected in torsion-resistant manner to a wheel of the vehicle to generate a brake force, to which end the gear unit is of a self-locking construction.
A brake arrangement having a wheel brake such as that known from DE 101 50 803 A1 is disadvantageous in that the electromechanical actuating unit provided for the parking brake system also has to be designed for load situations such as those for which very high brake forces have to be generated and maintained, for example to safely hold a fully loaded vehicle stationary on a road with a gradient of 30% and steeper. The result of this is that the technology of the electromechanical actuating unit has to be relatively complex, making the brake arrangement significantly more expensive.